Ahead of the unorthodox meeting scheduled for Friday to craft an agreement to "block Super PACs" from advertising in the U.S. Senate race in Massachusetts, Sen. Scott Brown and Democratic Challenger Elizabeth Warren each sent letters to each others campaigns on Wednesday, highlighting ideas about how to move forward.

They also both forwarded those letters to their media distribution lists minutes apart Wednesday evening.

Brown's proposal, which he has already signed, would require a candidate to spend 50 percent of the cost of a PAC ad on a charity of the other candidate's choice should an outside group air an ad in their favor.

Warren's letter says that any agreement should explicitly notify Super PACs and other third-party groups of the agreement and also cover sham ads, which could still hurt a candidate's finances depending on the angle of an ad.

Warren's condition would make it so a PAC supporting one candidate couldn't make an ad supportive of another just to hurt them financially in accordance with the agreement.

Both sides came to this point after Brown, for a second time, requested Warren join him in condemning the attack ads by Super PACs such as Crossroads GPS or the League of Conservation Voters. Warren previously joined Brown in condemning negative advertising, but stood by a third-party group's right to promote their interests in the Bay State.

Campaign managers from both Brown and Warren's camps are scheduled to meet on Friday to see if they can agree on a set of terms that would be the first agreement of its kind in relation to third-party advertising in U.S. elections.